Method for obtaining coatings on two-dimensional materials

Abstract

A method for obtaining a coating of a two-dimensional material on a surface, comprising the steps of: a) forming a dispersion by combining a layered material and at least one release agent in a first solvent; b) subjecting the dispersion formed in step a) to at least one treatment selected from ultrasonication, mechanical grinding and micronization to obtain a dispersion comprising a two-dimensional material; b1) Optionally, the dispersion comprising the two-dimensional material obtained in step b) is - extraction with solvents, - Centrifugation, -filtration, -concentrated, and c) applying the dispersion obtained in step b) or b1) to a surface by a method selected from the group consisting of spray application, application with a stainless steel spiral applicator (Mayer bar), dip coating, coating with rotary techniques (rotogravure, reverse roll, flexography and variations thereof), slot die coating and curtain coating to obtain a wet surface; d) drying the wet surface obtained in step c) to obtain a coating of the two-dimensional material on said surface; e) optionally repeating steps c) and d); Including, The method, wherein the dispersion does not include a binder or adhesion promoter.

Description

[Technical field]

[0001] The present invention relates to a method for obtaining a coating of a two-dimensional material on a surface. [Background technology]

[0002] For example, coatings of two-dimensional materials such as graphene, the basic unit of graphite, are known that are formed by stacking multiple layers of graphene held together by van der Waals interactions.

[0003] Graphene and other two-dimensional materials can be produced using a variety of different techniques, which can be divided into three main groups:

[0004] i) mechanical peeling, ii) chemical peeling, and iii) Atomic growth.

[0005] Within these groups there is a large amount of variation. i) Mechanical exfoliation using Scotch tape is a technique used by Nobel laureates Novoselov and Geim and remains the best technique for obtaining a pure, unoxidized form of graphene, also called "pristine" graphene. Unfortunately, this technique is only capable of producing a few sheets of graphene on silicon or silicon oxide, typically with dimensions less than 2000 square microns, and is expensive, limiting its application field to scientific research only.

[0006] Mechanical comminution (often using ball mills) of various raw materials such as graphite, pretreated bituminous pitch, and various kinds of carbonaceous products is used to obtain products at relatively low cost, up to a few hundred dollars per kilogram, but these products are rarely largely composed of a single layer. Often they are materials with a distribution of up to 10 layers, with the lateral dimensions of the thinnest products not exceeding 1 micron.

[0007] ii) Chemical exfoliation takes advantage of the structure of graphite, which is naturally composed of layers of graphene bonded to each other by weaker forces than the forces along the planes. This difference in bonding strength makes it possible, through the use of various chemical compounds, to separate one or more planes from each other without compromising the planar structure of graphene. Chemical exfoliation can be divided into two further groups:

[0008] ii.a) exfoliation that leaves the properties of graphene relatively unchanged; or ii.b) Exfoliation that alters these properties to a significant extent. In the latter case, it is called "Chemically Modified Graphene" (CMG).

[0009] ii.a) Among the methods that keep the properties of graphene relatively unchanged is exfoliation using solvents such as n-methyl pyrrolidone (NMP) and dimethylformamide (DMF), which, with the aid of an ultrasonication process, makes it possible to obtain lateral dimensions in the micron range for a significant proportion of monolayer material. The drawback is that this type of solvent is difficult to remove from the graphene surface.

[0010] Another technique is the intercalation of chemical compounds and subsequent exfoliation by thermal expansion of the intercalated graphite. A final mechanical treatment produces graphene powder. This technique therefore combines the advantages of chemical exfoliation and mechanical action. In this case, larger lateral dimensions are obtained than with mechanical exfoliation, but the technique is more complex and it is unclear whether the properties of graphene remain unchanged.

[0011] ii.b) Among the techniques for producing CMG there are methods that result in significant lateral dimensions up to 300 microns and have the material almost entirely in a monolayer structure. This is the Hummers exfoliation method and some of its variants known to those skilled in the art. Unfortunately, the result of this process is not graphene but its oxidized form, graphene oxide (GO). This material retains good mechanical properties but completely loses its electrical properties and becomes completely insulating.

[0012] The inventors of the present invention have previously devised a method for producing a carbon-based nanostructured material (WO2014033274) in which graphene is formed from less than 10 monolayers, has a length to thickness ratio greater than 10, or even greater than 100, and can have an electrical conductivity greater than 5000 S / m.

[0013] The deposition of thin coatings is a rapidly growing industry sector due to several advantageous practical aspects: it reduces the cost of materials, reduces the size of manufactured articles, provides new properties not possible with bulk materials, and speeds up manufacturing processes.

[0014] On an industrial scale, the most commonly used processes for the production of thin layers can be divided into: Chemical method: -Electroplating, - Chemical baths, -Chemical Vapor Deposition (CVD) and Plasma Enhanced Chemical Vapor Deposition (PECVD), -Atomic Layer Deposition (ADL), Molecular Layer Deposition (MDL), and Physical method: -Physical Vapor Deposition (PVD) and Cathodic Arc Variation (arc-PVD), -Molecular Beam Epitaxy (MBE), - sputter deposition, and -Electrospray deposition.

[0015] Most of these techniques use industrially expensive conditions, such as high temperatures, systems operating under vacuum, high energy consumption, or discontinuous, slow processes.

[0016] Simple coating techniques such as spray application, application with stainless steel spiral applicators (Mayer bars), dip coating, coating with rotary techniques (rotogravure, reverse roll, flexography, and their variations), slot-die coating, and curtain coating exist but currently require the use of binders and adhesion promoters.

[0017] Commonly used binders are oligomeric or polymeric products, polyurethanes, polyesters, acetates, silanes, oils, and acrylic, alkyd or epoxy resins.

[0018] Binders, when used to deposit 2D materials, usually impair the properties of those materials, not only because they have completely different chemical / physical properties, but also because they increase the distance between adjacent sheets and therefore the resulting contact resistance.

[0019] In particular, the presence of binders in the graphene adversely affects several properties of the coating in the following ways:

[0020] -Reduces electrical conductivity, -Reduces thermal conductivity, -contributes to the coating thickness, making it difficult, if not impossible, to obtain coatings with thicknesses below 20 nm. Summary of the Invention [Problem to be solved by the invention]

[0021] In light of the abovementioned drawbacks, the object of the present invention is to provide a simple method for producing coatings of two-dimensional materials having advantageous properties, in particular with regard to electrical conductivity, thermal conductivity and tribological properties.

[0022] Within this objective, it is an object of the present invention to provide an economically advantageous method which makes it possible to obtain thin coatings of two-dimensional materials.

[0023] Another object of the invention is to provide a coating of a two-dimensional material having properties that allow its use, for example, for the production of conductive inks, conductive films, antistatic films, additives for plastic materials, sensors, transistors, diodes, light modulators, transparent conductive materials, solar cells, batteries, ultracapacitors, media for medical or biomedical active ingredients, antibacterial substances, protective coatings, lubricating coatings, lubricants, paints, flexible electronics, metal and polymer composites or fillers for composites. [Means for solving the problem]

[0024] This object, as well as these and other objects that will become more apparent hereinafter, are achieved by a method for obtaining a coating of a two-dimensional material on a surface, the method comprising: a) forming a dispersion by combining a layered material and at least one release agent in a first solvent; b) subjecting the dispersion formed in step a) to at least one treatment selected from ultrasonication, mechanical grinding and micronization to obtain a dispersion comprising a two-dimensional material; b1) Optionally, the dispersion comprising the two-dimensional material obtained in step b) is - extraction with solvents, - Centrifugation, -filtration, -concentrated, and c) applying the dispersion obtained in step b) or b1) to a surface by a method selected from the group consisting of spray application, application with a stainless steel spiral applicator (Mayer bar), dip coating, coating by a rolling technique, slot die coating, and curtain coating to obtain a wet surface; d) drying the wet surface obtained in step c) to obtain a coating of the two-dimensional material on said surface; e) optionally repeating steps c) and d); Including, The dispersion does not include a binder or adhesion promoter.

[0025] The object of the invention is also achieved by a coating of a two-dimensional material obtainable according to the method of the invention.

[0026] The objects and aims of the present invention are finally achieved by the use of the coating according to the invention for the production of conductive inks, conductive films, antistatic films, additives for plastic materials, sensors, transistors, diodes, light modulators, transparent conductive materials, solar cells, batteries, ultracapacitors, media for medical or biomedical active ingredients, antibacterial substances, protective coatings, lubricating coatings, lubricants, paints, flexible electronics, metal and polymer composites or fillers for composites.

[0027] Further features and advantages of the present invention will become more apparent from the following detailed description.

[0028] In a first aspect, the present invention relates to a method for obtaining a coating of a two-dimensional material on a surface, the method comprising the steps of: a) forming a dispersion by combining a layered material and at least one release agent in a first solvent; b) subjecting the dispersion formed in step a) to at least one treatment selected from ultrasonication, mechanical grinding and micronization to obtain a dispersion comprising a two-dimensional material; b1) Optionally, the dispersion comprising the two-dimensional material obtained in step b) is - extraction with solvents, - Centrifugation, -filtration, - Sedimentation, and c) applying the dispersion obtained in step b) or b1) to a surface by a method selected from the group consisting of spray application, application with a stainless steel spiral applicator (Mayer bar), dip coating, coating by a rolling technique, slot die coating, and curtain coating to obtain a wet surface; d) drying the wet surface obtained in step c) to obtain a coating of the two-dimensional material on the surface; e) optionally repeating steps c) and d); Including, The dispersion does not contain a binder or adhesion promoter.

[0029] Preferably, the mixing in step a) is carried out at a temperature comprised between −10° C. and 160° C., within the limits of the solvent used.

[0030] The methods used in step b) are known to those skilled in the art. In the present invention, "micronization" refers to a comminution technique in which the fragments obtained have micrometer and submicrometer dimensions. Among the micronization techniques is, for example, the use of high-pressure fluid jets in a "jet mill".

[0031] Non-limiting examples of materials that can be used to obtain a two-dimensional coating in the method of the invention described above are Examples 1 to 12 of WO2014033274.

[0032] Some applications require high purity and dimensional uniformity of the coating material. In these cases, the dispersion obtained in step b) is further processed with one or more additional techniques selected from extraction with a solvent, centrifugation, filtration, and concentration (step b1).

[0033] The application of the dispersion obtained in step b) or b1) can be carried out by the above-mentioned techniques known to the person skilled in the art. Spin-coating techniques include rotogravure, reverse roll, flexographic printing and their variants.

[0034] The coating obtained by the above method can be further stabilized by one or more washes with a solvent.

[0035] In a preferred method of carrying out the method of the invention, the method further comprises a step f) of washing the coating of the two-dimensional material obtained in step d) or e) with a second solvent selected from the group consisting of alcohols, aldehydes, ketones, esters, aliphatic hydrocarbons, aromatic hydrocarbons, chlorinated hydrocarbons, and mixtures thereof.

[0036] One or more further additives selected from the group consisting of dispersants, wetting agents, biocides, binders, carbon nanotubes (CNTs), fullerenes, metals, graphene derivatives, antioxidants, polymers, dyes, and rheological additives can be added to the dispersion prior to step c).

[0037] Preferably, the starting layered material is selected from the group consisting of graphene, boron nitride, molybdenum disulfide, transition metal dichalcogenides, graphite and graphitic materials, phosphorene, and xenon.

[0038] In a preferred method of carrying out the method of the present invention, the at least one release agent is selected from the group consisting of oligomers or polymers comprising 1 to 1000 repeating units, preferably 3 to 20 repeating units, comprising at least one ether group and at least one aryl group substituted with one or more functional groups selected from the group consisting of aromatic amines, aromatic amides, aromatic imines, aromatic sulfites, aromatic compounds of phosphorus, aromatic carboxylic acids, phenols, aromatic alcohols, aromatic ethers, aromatic aldehydes, aromatic esters, aromatic anhydrides, nitroaromatic groups, pyridines, pyrimidines, imidazoles, azobenzenes, anthracenes, diphenyls, aromatic halides, alkanes and alkenes.

[0039] In another preferred method of carrying out the method of the present invention, the at least one release agent is selected from the group consisting of one or more of polyvinyl alcohol (PVA), polyethylene oxide (PEO), polyoxyethylene (POE), polyethylene glycol (PEG), polypropylene glycol, carrageenan and polyvinyl amine (PVAm).

[0040] In a preferred method of carrying out the method according to the invention, the at least one stripping agent is selected from the group constituted by one or more aryl ethoxylates, preferably one or more of nonylphenyl ethoxylate and octylphenyl ethoxylate in solution.

[0041] Preferably, the at least one release agent is present in an amount of from 0.01% to 80% by weight, more preferably from 0.03% to 40% by weight, even more preferably from 0.1% to 20% by weight, relative to the total weight of the dispersion.

[0042] Preferably, the layered material in the dispersion of step a) represents 0.5% to 80% by weight relative to the total weight of the dispersion.

[0043] Preferably, the first solvent used to form the dispersion in step a) is selected from the group constituted by water, alcohols, esters, aldehydes and ketones.

[0044] The properties of the coating obtained by the method of the invention, such as, for example, electrical conductivity, thermal conductivity, optical properties, lubrication properties, radiation absorption, surface tension, adhesion, influence on electromagnetic radiation, influence of electrical or opto-electrical interactions with other material layers, can be further improved by further steps of surface cleaning and activation. In particular, the removal of residues that previous treatments may leave on the surface of the two-dimensional material is advantageous.

[0045] In a preferred method of carrying out the method of the invention, the method further comprises a step g) of subjecting the surface coated with the two-dimensional material obtained in step d), e) or f) to one or more mechanical treatments selected from the group consisting of UV (Ultraviolet) radiation, IR (Infrared) radiation, plasma, radiation, surface cleaning, brushing, lapping and polishing.

[0046] In a second aspect, the present invention relates to a coating of a two-dimensional material obtainable according to any of the above-mentioned methods implementing the method of the invention.

[0047] The method of the invention advantageously makes it possible to obtain thin layers in a simple, continuous, low technology process, without the need to use processes operating at high temperatures or under vacuum.

[0048] Furthermore, the method of the invention advantageously does not require the use of binders or adhesion promoters commonly used to produce coatings, resulting in benefits such as the possibility of producing thin layers while maintaining the properties of two-dimensional materials, such as electrical conductivity, thermal conductivity, lubricating properties, and optical properties.

[0049] If the dispersion is applied at a low concentration of layered material, for example 0.01% by weight relative to the total weight of the dispersion, and the solvent is then evaporated, a thin coating of up to 1.0 nm is produced. It is possible to vary the concentration or number of thin layers deposited in order to obtain a coating of 1.0 to 2000 nm, preferably 10 to 500 nm (expressed as the thickness of the dry material).

[0050] In a preferred embodiment, the coating has a thickness of 20 nm or less. The process of the invention makes it possible to obtain a coating characterized by:

[0051] -Conductivity included from 0.01 ohms / square to insulation level, measured according to ASTM F1711-96(2016), IEC61340-4-1 and IEC61340-4-5 standards; - Thermal conductivity between 0.1 W / mK and 2000 W / mK, measured using transient thermal reflectance according to the ISO / TTA4:2002 standard; - a coefficient of friction of at least 0.01μ, measured according to the ASTM G99-17 standard; - adhesion to the substrate, measured by cross-cut test up to 5B according to ASTM D3359 method; - Hardness up to 6H, measured according to ASTM D3363-20 standard.

[0052] The method of the invention therefore makes it possible to maintain the initial properties of the exfoliated material, i.e., electrical conductivity, thermal conductivity and tribological properties in the case of graphene, and thermal conductivity, radiation shielding and tribological properties in the case of boron nitride.

[0053] In a third aspect the present invention relates to the use of the coating obtained using the method of the present invention for the production of conductive inks, conductive films, antistatic films, additives for plastic materials, sensors, transistors, diodes, light modulators, transparent conductive materials, solar cells, batteries, ultracapacitors, media for medical or biomedical active ingredients, antibacterial substances, protective coatings, lubricating coatings, lubricants, paints, flexible electronics, metal and polymer composites or fillers for composites. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0054] The invention will now be described with reference to the following non-limiting examples. Example 1: Preparation and characterization of graphene coatings according to the invention 50 g of graphite in powder form with particles up to 120 microns were mixed with 200 ml of a 0.1% solution of nonylphenylethoxylate (Sigma-Aldrich) for 30 seconds at 40 °C to obtain a dispersion. The dispersion was then ground for 1 hour at a temperature of 40 °C using a ball mill using balls with a diameter of 0.3 to 1.2 mm. The mixture thus obtained was then centrifuged at 5400 rpm for 4 minutes to remove the unground graphite. The dispersion was heated to 90 °C to concentrate the exfoliated graphene. After cooling, the top part (with a low concentration) was removed and the concentrated solution was characterized using gravimetric analysis giving a solid concentration of 25 mg / ml. The dispersion thus obtained was sprayed with a nozzle spray onto a polyester film until the film was completely covered. The film containing the graphene solution was then dried with an IR lamp and subsequently washed with acetone and air-dried.

[0055] The graphene coated film was characterized for electrical conductivity using a Warmbier SRM200 surface resistivity meter, using a four-point analysis system according to ASTM F1711-96 (2016) standard. The electrical conductivity was found to be 20M Ohm / sq. IEC 61340-4-1, IEC 61340-4-5.

[0056] The film was then cleaned by rubbing the surface with paper and the measurement of surface resistance was repeated, going from 20M ohms / square to 700 ohms / square. The adhesion to the substrate was measured by cross-cut test and a value of 5B was found according to the ASTM D3359 standard method.

[0057] Example 2: Preparation and characterization of a coating of boron nitride according to the invention 0.1 grams of boron nitride was added to 8 ml of a solution of water and 3% polyvinyl alcohol (PVA) (Sigma-Aldrich) and mixed for 20 seconds at 60°C. The dispersion was sonicated for 20 hours at a temperature of 60°C with a Fisher Scientific sonicator, FB15047 30W 37KHz. The dispersion was centrifuged for 30 minutes at 3800 rpm. The concentration was determined gravimetrically to be 1.5mg / ml. 4 ml of the dispersion was spread on a silicon (p) wafer with a Mayer bar. Once the solution was dry, it was washed with a 1:1 mixture of isopropanol and ethyl acetate. The silicon wafer was analyzed using Modulated Photothermal Radiometry (MPTR) technique, 3ω method, transient thermal reflectance, ISO / TTA4:2002, and found to have a thermal conductivity of more than 50W / mK.

[0058] Example 3: Preparation and characterization of graphene and molybdenum disulfide coatings according to the present invention 3 kg of graphite flakes were ground together with 300 grams of molybdenum disulfide in a ball mill containing 1.5 to 0.3 mm balls and 20 kg of a 5% mixture of styrenated phenol ethoxylate (Chemical China) and polyvinylamine in a 10:1 ratio. After 24 hours of grinding, the mixture was filtered through a 5 micron mesh filter. The dispersion thus obtained was used to coat a steel disc by immersion (100Cr6) (EN ISO683-17). After drying, two more coatings were performed by immersion in the dispersion followed by drying cycles. The disc was then washed with perchloroethylene and then subjected to pin-on-disc tribological analysis using the ASTM G99-17 method, finding a friction coefficient of 0.06μ.

[0059] In practice, it has been found that the method according to the invention fully achieves the set objective of making it possible to obtain in a simple and economical manner coatings of two-dimensional materials that maintain the properties of the exfoliated starting material, without the drawbacks associated with the use of binders from an energy point of view. The method according to the invention furthermore advantageously makes it possible to obtain thin layers having a thickness of less than 20 nm.

[0060] The method thus conceived is susceptible to numerous modifications and variations, all of which are within the scope of the appended claims. Moreover, all the details may be replaced by other technically equivalent elements.

[0061] In practice, the materials and dimensions used can be any depending on the requirements and the state of the art.

[0062] This patent application is in part based on the results of projects funded by the H2020 research and innovation program, namely Graphene Flagship Grant Agreement No. 881603, and Graphene Flagship No. 649953, and Marie Skłodowska-Curie Grant Agreement No. 956923-StibNite.

[0063] The disclosures of Italian Patent Application No. 102022000009011, from which this application claims priority, are incorporated herein by reference.

Claims

1. A method for obtaining a coating of a two-dimensional material on a surface, comprising the steps of: a) forming a dispersion by combining a layered material and at least one release agent in a first solvent; b) subjecting the dispersion formed in step a) to at least one treatment selected from ultrasonication, mechanical grinding and micronization to obtain a dispersion comprising a two-dimensional material; b1) optionally, the dispersion comprising the two-dimensional material obtained in step b), - extraction with solvents, - centrifugation, -filtration, -concentrated, and c) applying said dispersion obtained in step b) or b1) to a surface by a method selected from the group consisting of spray coating, coating with a stainless steel spiral applicator (Mayer bar), dip coating, coating with rotary techniques (rotogravure, reverse roll, flexography and variations thereof), slot die coating and curtain coating to obtain a wet surface; d) drying the wet surface obtained in step c) to obtain a coating of two-dimensional material on said surface; e) optionally repeating steps c) and d); Including, The method, wherein the dispersion does not include a binder or adhesion promoter.

2. 2. The method of claim 1, further comprising a step f) of washing said coating of two-dimensional material obtained in step d) with a second solvent selected from the group consisting of alcohols, aldehydes, esters, ketones, aliphatic, aromatic and chlorinated hydrocarbons, and mixtures thereof.

3. 3. The method according to claim 1 or 2, wherein one or more additives selected from the group consisting of dispersants, wetting agents, biocides, binders, carbon nanotubes (CNTs), fullerenes, metals, graphene derivatives, antioxidants, polymers, dyes and rheological additives are added to the dispersion before step c).

4. 2. The method of any one of the preceding claims, wherein the layered material is selected from the group consisting of graphene, boron nitride, molybdenum disulfide, transition metal dichalcogenides, graphite and graphitic materials, phosphorene, and xenon.

5. The at least one release agent is a) an oligomer or polymer comprising 1 to 1000 repeating units, preferably 3 to 20 repeating units, comprising at least one ether group and at least one aryl group substituted with one or more functional groups selected from the group consisting of aromatic amines, aromatic amides, aromatic imines, aromatic sulfites, aromatic compounds of phosphorus, aromatic carboxylic acids, phenols, aromatic alcohols, aromatic ethers, aromatic aldehydes, aromatic esters, aromatic anhydrides, nitroaromatic groups, pyridines, pyrimidines, imidazoles, azobenzenes, anthracenes, diphenyls, aromatic halides, alkanes and alkenes; or b) the method of any one of the preceding claims, selected from the group consisting of one or more of polyvinyl alcohol (PVA), polyethylene oxide (PEO), polyoxyethylene (POE), polyethylene glycol (PEG), polypropylene glycol, carrageenan, and polyvinylamine (PVAm).

6. 10. The method according to any one of the preceding claims, further comprising a step g) of subjecting the surface coated with the two-dimensional material obtained in step d), e) or f) to one or more of the following mechanical treatments selected from the group consisting of ultraviolet (UV) radiation, infrared (IR) radiation, plasma, radiation, surface cleaning, brushing, lapping and polishing.

7. 2. The method of any one of the preceding claims, wherein the first solvent is selected from the group consisting of water, alcohols, aldehydes, esters, and ketones.

8. 8. The method according to any one of claims 1 to 7, wherein the at least one stripping agent is selected from the group constituted by one or more aryl ethoxylates, preferably aryl ethoxylates comprising one or more nonylphenyl ethoxylates and octylphenyl ethoxylates in solution.

9. A coating of a two-dimensional material obtainable according to the method according to any one of claims 1 to 8.

10. 10. Use of the coating according to claim 9 for the production of conductive inks, conductive films, antistatic films, additives for plastic materials, sensors, transistors, diodes, light modulators, transparent conductive materials, solar cells, batteries, ultracapacitors, media for medical or biomedical active ingredients, antibacterial substances, protective coatings, lubricating coatings, lubricants, paints, flexible electronics, metal and polymer composites or fillers for composites.

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